1. Field of the Invention
The present invention relates to current monitoring circuits and systems. More specifically, the present invention relates to analog circuits and systems for monitoring current in electrically isolated circuits.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
For many applications, there is a need to determine flow of current in a circuit or conductor. The conventional approach involves the insertion of a series resistor in the conductor. However, the use of a resistor is problematic, lossy and impractical in applications wherein an above-ground voltage is present on the current carrying conductor. More importantly, in some applications, the use of the resistor substantially interferes with the intended operation of the circuit. Hence, for these applications, there is a need for an indirect (or isolated) method for monitoring the current flow in the conductor.
One conventional isolated current monitoring scheme involves use of Hall effect devices. A Hall effect device is a device, which is magnetically coupled to the current carrying conductor. In order to accomplish the required magnetic coupling, the Hall effect device must be placed in series with the magnetic path which encircles the conductor. The Hall effect device has a typical thickness of 0.04 inch. Since the Hall device has magnetic properties similar to those of air, an "air" gap is effectively placed in the magnetic path. The low-permeability gap reduces the overall permeability of the magnetic circuit, to assure that saturation of the magnetic core is avoided. The inductor, thus formed, is effectively inserted in series with the current-carrying conductor of the current monitoring device. For some applications, this added inductance is a circuit parasite capable of storing energy. Most Hall effect devices are prone to relatively large drifts in output when subjected to temperature changes. Complex, and often troublesome, circuitry is frequently required to overcome this temperature drift problem.
Another approach involves the use of a transformer to indirectly sense current flow in the monitored circuit. The transformer provides a coil (typically a single-turn primary) to pick up energy in the magnetic field created by the flow of current through the monitored conductor or circuit.
Transformers are often used for this purpose inasmuch as: 1) the magnetic coupling thereof provides electrical isolation from the circuit being monitored, 2) when large currents are to be measured, the use of a single-turn primary and multi-turn secondary, reduces power loss from the circuit being monitored, and 3) proper choice of the transformer turns ratio provides improved signal-to-noise ratio for increased accuracy and trouble-free operation.
Despite these advantages, conventional transformer based current monitoring circuits cannot be utilized to measure current in DC (direct current) circuits or in those switched circuits that employ duty ratios substantially greater than 50%. This is due to the limitation that transformers fail to function when the magnetic cores thereof are saturated. Hence, conventional transformer based current monitoring circuits provide for a transformer core "reset" during a required OFF portion of every cycle to ensure that the transformer does not "walk" into saturation.
Thus, there is a need in the art for a current monitoring circuit that allows for the measurement of DC and high duty cycle currents without interfering with the operation thereof.